White nylon plastic with a matte finish and slight grainy feel.
About this Product
Less plastic = less cost. Lego compatible Arduino UNO mount with minimalist design. If you want something a little fancier and with more mounting possibilities try the Deluxe version for just a few bucks more.
I have only tried this part in the White, Strong Flexible material. It connects securely to both the Arduino UNO and to other Lego bricks while still being removable without undue effort. Other materials may work better or worse, if you try a different material, please let me know how it works out.